The present invention relates to a modular system useable for offshore production, storage and/or loading of hydrocarbons.
Oil production at sea is normally carried out from production platforms connected to the land by pipe lines.
If the production of the field, quantity or duration, is insufficient to offset the cost of a pipe line connecting the platform to the coast, a platform must be provided for production and storage at sea.
Such systems, with a view to the production of so called marginal fields, have been developed.
One of these systems consists in using a semi submersible drilling platform converted for production.
The deck of the drilling platform is freed of drilling equipment, this being replaced by production equipment.
With the crude oil separated into three components (oil, water, gas), the oil is stored in a tanker permanently anchored on the field, by means of a buoy so that the tanker can constantly take up a position in the axis of the wind.
A second buoy is generally provided which serves as sea terminal and which allows a second tanker to shuttle between the field and the coast.
A second production system consists in using a tanker specially converted for production.
In this case, the same floating support serves for production (the equipment being placed on the deck) and for storage.
As in the above case, the tanker is permanently anchored to a buoy and a second buoy serves as sea terminal for unloading the oil into a second tanker which shuttles between the field and the coast.
These systems have two drawbacks:
(a) the necessity of transforming floating supports not provided for sea production and so not always adapted to requirements from the technical and cost points of view;
(b) limitation of the depth of water in which these systems may be used. These two systems are in effect dependent on the possibility of anchoring conventional buoys, which limits these systems in practice to water depths less than a 150 m (see, for example, the article "An analysis of tanker--Based floating production systems for small offshore fields", by Messrs. W. R. Leod and L. H. Sumudlers, Journal of Petroleum Technology, August 1982, page 1871 to 1879).
The prior art may be illustrated by U.S. Pat. Nos. 3,434,442 and 4,234,270 as well as by German patent application Nos. 2.701.242 and 2.727.082.
An essential object of the present invention is to provide a floating production system comprising integrated storage, this system further forming the sea terminal and being able to be anchored in depths greater than those of conventional buoys.
Another important object of the invention is to provide a modular structure easily adaptable to requirements.
These objectives are attained, in accordance with the invention, with a floating modular system able to serve more especially for the offshore production, storage and/or loading of the hydrocarbons, comprising an assembly of cylinders connected rigidly together, which comprise in combination:
(a) at least one metal floatation cylinder, adapted to occupy a vertical position in use, the bottom of said floatation cylinder being situated below the level of the water and the top of said cylinder emerging above the level of the water and supporting a deck or platform with production and living equipment, said floatation cylinder being firmly secured to several metal cylinders descending possibly below the bottom of this floatation cylinder and comprising
(b) metal cylinders for storing the oil, this storage being effected on a water column connected to the water surrounding the system, the level of the water in the cylinders lowering or rising depending on whether oil is stored or withdrawn, said storage cylinders being preferably entirely below the level of the surrounding water and
(c) metal ballast cylinders or cylinder parts which may be more especially filled with oil, water, air or inert gas and associated regulation means adapted to compensate for the floatability variation of the system following variations of the water-oil level in the storage systems.
The storage and ballast cylinders may be joined to one another or not.
It should be understood that the term cylinder is to be understood in its widest sense and not only in the sense of a cylinder of revolution although this form is convenient to construct.
According to a particular embodiment, the storage and ballast cylinders are disposed at least around the floatation cylinder which will then occupy a central position. In this case, the storage and ballast cylinders are termed peripheral cylinders and the floatation cylinder is termed central cylinder.
In another particular embodiment, the metal floatation cylinder will not be used and only the assembly formed of the storage and ballast cylinders disposed possibly about an axis will be kept. Floatation may then be provided by one or more floats connected more especially directly to said assembly or by the positive buoyancy of some at least of the storage or ballast cylinders. In this particular case, the platform or the deck may be connected directly to the assembly by assembly legs.